This invention relates generally to compositions of matter and methods for the preparation of composite and hybrid polymers and resins in which at least one component is a chemically modified carboxylate-alumoxane. The carboxylate-alumoxanes are chemically bonded into the polymer backbone through reaction of the appropriate functional groups of a polymer precursor with the carboxylate-alumoxane. The method of the present invention can be used to produce polymers with organic and inorganic backbones. The polymers produced according to the invention can be either a thermoset or a thermoplastic and can be prepared by either a condensation polymerization reaction or an addition polymerization reaction. The invention provides for the formation of carboxylate-alumoxane materials prepared by the reaction of carboxylate-alumoxanes with polymer precursors including without limitation: epoxides, phenol-formaldehyde resins, polyamides, polyesters, polyimides, polycarbonates, polyurethanes, quinone-amine polymers, and acrylates.
Condensation polymers are an important class of polymers that are prepared by the reaction of low molecular weight precursors that contain reactive functional groups such as amines, hydroxyls, acid chlorides, anhydrides and carboxylic acids. Polymeric materials prepared by condensation polymers include (but are not limited to) epoxies, phenol-formaldehyde resins, polyurethanes, polyamides, polyesters, polyimides, polycarbonates, quinone-amine polymers and polysulfones. These materials possess a range of properties and their applications are widespread.
Another name for the class of materials known as condensation polymers are thermoset polymers. Thermosettable polymers, in general, exist as liquids that, upon heating, undergo reactions to form solid, highly cross-linked polymer matrices. Once formed, thermoset polymers cannot be reformed into different shapes by heating.
In general, unfilled thermoset polymers tend to be harder, more brittle and not as tough as thermoplastic polymers. Thus, it is common practice to add a second phase (i.e., fillers) to thermosetting polymers to improve their properties. In addition, incorporation of fillers into the polymer matrices also strengthens and stiffens the polymer matrix allowing the polymers to be used in an expanded range of structural applications.
A common technique for improving the properties of thermoset resins is the use of inorganic fillers (Handbook of Fillers and Reinforcements for Plastics 11-58; Katz, H. J. and Milewski, J. V., eds.; Van Nostrand Reinhold Co.; New York). Inorganic fillers impart a number of desirable mechanical and barrier properties to the polymer. These properties include improved tensile strength, stiffness, abrasion resistance, dimensional stability and barriers to gases, solvents and water-vapor. As a second phase, the inorganic fillers can also affect other polymer properties such as pot life, cure exotherms, shrinkage, thermal conductivity, thermal shock resistance, heat deflection temperature, machinability, hardness, compressive strength, flexural strength, impact strength and electrical conductivity. The properties of polymers can also be modified by incorporation of a second organic or polymer phase into the thermoset polymers. For example, epoxy resins are routinely toughened (strengthened) by the incorporation of an elastomeric polymer.
The extent to which the inorganic fillers affect polymer properties is closely associated with the volume fraction of filler incorporated into the polymer, the particle size of the filler and the degree to which the filler is bound to the polymer matrix. Thus, the properties of the filler-modified polymers depend on the size, shape and dispersion uniformity of the inorganic modifier, as well as the degree of interaction between the inorganic modifier and the organic matrix. Therefore, the best performance is achieved with inorganic fillers consisting of small particles that are uniformly dispersed throughout the polymer and interact strongly with the organic matrix.
Until recently, the particle sizes of fillers used to improve polymeric properties have been on the micron length scale or even larger. These dense fillers have relatively low surface areas. The surface area of a filler is one of its more important properties, as the surface area determines the amount of contact and bonding between the polymer and the filler (Katz and Milewski, 1978). The size of the particles also determines the volume fraction that can be obtained with a given filler at a given weight loading. Most of the properties (mechanical and barrier) of filled polymers are directly related to volume fraction (and correspondingly to the particle size) of the fillers.
The greatest effect of fillers on the properties of polymer-filler composites appears to occur for fillers possessing dimensions on the nanometer length scale. Nano-particles are ordinarily defined as materials with sizes ranging from 1 nm to 1 xcexcm. Nano-particles have higher surface areas and at the same weight have higher volume loadings than do larger particles. The total sum of the interactions between filler particles and the polymer are larger for nanometer sized particles than for larger particles. In addition, smaller particles produce smaller stress concentrations in the composite material. Unfortunately, the handling and dispersing of nanometer sized particles can be difficult; for example, small particles rapidly build up large static charges that can lead to the formation of hazardous breathable dusts. Additionally, inorganic oxides are hydrophilic, while most polymers are hydrophobic. This leads to segregation of the two phases and agglomeration of the powders resulting in a decrease in the overall performance of the polymer composite. Hence, it is desired to provide a technique for incorporating advantageous nanoparticle fillers into condensation polymers.
Like condensation polymers, addition polymers are an increasingly important class of polymers and have widespread applications. Addition polymers are prepared by the reaction of a monomer with an unsaturated group. Industrially, addition polymerizations are typically carried out in one of four general ways: in bulk, in solutions, in suspensions, or in emulsions. For the bulk case, only a monomer and polymerization initiator are reacted exothermically to produce polymers such as polystyrene, or poly(vinyl chloride). While this process is usually difficult to control and generates a lower yield polymer, it does have the advantages of producing polymers with high optical clarity and low contamination.
Solution polymerization is similar to bulk polymerization with the simple addition of a solvent medium. The advantage of solution polymerization is that the solvent allows for heat transfer during the reaction, which results in a much higher yield of polymer. The disadvantage of solution polymerization is that solvents must be chosen carefully, as side chain reactions can occur. Solution polymerization, however, is used frequently to polymerize such monomers as vinyl acetate and ethylene. Suspension polymerization, while utilizing a solvent, is much more akin to the bulk polymerization method rather to the solution method, in that droplets of the monomer are suspended in a carrier in which the monomer is insoluble. The carrier allows the advantageous transfer of heat, and the occurrence of unwanted side reactions is drastically diminished. Finally, emulsion polymerization is similar to suspension polymerization with two important differences: (1) the monomer droplet size is smaller, and (2) the initiator is insoluble in the monomer, but soluble in the carrier. This method is chiefly used to polymerize acrylics, poly(vinyl acetate), and numerous other copolymers. The advantage to emulsion polymerization is that the chain length of the monomer can be controlled without regard to reaction rates.
These different polymerization methods are based on several features associated with addition reactions: (1) the mass of the polymer decreases with increasing temperature and reaction rate, (2) the longer the reaction time, the more monomer is consumed and consequently, the greater the polymeric yield, (3) the reaction time and the molecular weight of the synthesized polymer are unrelated, and (4) the highest molecular weight polymer possible is formed immediately after the reaction begins and does not vary during the remainder of the reaction time. Although the incorporation of fillers into thermoplastic polymers does not provide the same advantages that it does for thermoset polymers, there are nonetheless instances where it is desirable to incorporate fillers into thermoplastic polymers.
In both condensation and addition polymerizations, the performance of the polymer-filler compound is strongly dependent on the strength of the interaction between polymer and filler. There are a number of interactions that can occur between the polymer and the inorganic filler, including van der Waals and dipole-dipole interactions, hydrogen bonding and covalent bonding. The weakest interactions between the polymer and the filler are the van der Waals and dipole-dipole interactions, with hydrogen bonding affording the next strongest interaction. The strongest interaction is obtained through covalent bonding. The polymer-filler covalent bonding interactions are on the order of 30 to 100 times greater than those that can be obtained by hydrogen bonding (G. Whitesides, et al; Articles; Molecular Self-Assembly and Nanochemistry: A Chemical stratgy for the Synthesis of Nanostructures; 254 Science 1312-1319 (November 1991)). Covalent bonding of the filler particles to the polymer lattice allows better transfer of mechanical loads to the particles, thereby improving the mechanical properties of the resulting polymer-filler compounds. Covalent attachment of the particles to the polymer matrices can also promote toughening of the polymer composite, as numerous bonds must be broken before cracks can propagate through the polymer. It is therefore desirable to identify small size (i.e., nanometer scale) inorganic fillers that can be readily incorporated into polymer matrices through covalent bonds.
Ceramic materials have excellent mechanical properties, such as heat-resistance, wear-resistance and strength, however, they are typically brittle and difficult to form into complex shapes. In addition, the rational chemical design of new ceramics (and inorganic materials in general) is poorly understood (A. MacInnes, et al; Indium tert-butylthiolates as Single Source Precursors for Indium (II) Sulfide Thin Films: Is Molecular Design Enough; 449 Journal of Organometallic Chemistry 95 (1993)). In contrast, organic polymers and resins are readily processed by a variety of methods. Possibly the most important property of organic polymers is the ease with which their physical properties may be modified through synthetic chemistry.
We have previously reported that aluminum-oxide nanoparticles (5 to 80 nm) can be prepared by the reaction of the mineral boehmite with carboxylic acids. The identity of the carboxylic acid appears to control the size of the nanoparticles. These materials are termed carboxylate-alumoxanes and may be prepared with an almost limitless variety of functional groups.
The aluminum based sol-gels (Yoldas, B; Alumina Gels that Form Porous Transparent Al2O3; Journal of Material Science 1856-1860 (1975)) formed during the hydrolysis of aluminum compounds belong to a general class of compounds, namely alumoxanes. These materials were first reported in 1958 (Andrianov, K. et al.; Synthesis of New Polymers with Inorganic Chairs of Molecules; 30 Journal of Polymer Science 513-524, (1958)) with siloxide substituents, however, they have since been prepared with a wide variety of substituents on aluminum. Recent work has shown that the structure of alumoxanes resembles a three dimensional cage (Apblett, A. et al. Synthesis and Characterization of Triethylsiloxy-Substituted Alumoxanes: Their Structural Relationship to the Minerals Boehmite and Diaspore; American Chemical Society 167-181 (1992) (hereinafter xe2x80x9cApblett et al. (1992)xe2x80x9d) and (Landry, C. et al.; Siloxy-Substituted Alumoxanes: Synthesis from Polydialkylsiloxanes and Trimethylaluminum, and Applications as Aluminosilicate Precursors; Journal of Materials Chemistry 597-602 (1993)). For example, siloxy-alumoxanes, [Al(O)(OH)x(OSiR3)1xe2x88x92x]n, consist of an aluminum-oxygen core structure (FIG. 1) analogous to that found in the mineral boehmite, [Al(O)(OH)]n, with siloxide substituents surrounding the core. In the siloxy-alumoxanes, the xe2x80x9corganicxe2x80x9d substituent typically resembles that shown in FIG. 2. However, the carboxylate anion, [RCO2]xe2x88x92, is an isoelectronic and structural analog of the organic portion found in the siloxy-alumoxanes and is known to act as a bridging ligand across two aluminum centers (FIG. 3), (Koide, Y. et al.; Alumoxanes as Cocatalysts in the Palladium-Catalyzed Copolymerization of Carbon Monoxide and Ethylene: Genesis of a Structure-Activity Relationship; 15 Organometallics 2213-2226 (No. 9, 1996)) and Koide, Y. et al.; [Al5(Bu)5(xcexc3-O)2(xcexc-OH)2(xcexcO2CPh)2]: A Model for the Interaction of Carboxylic Acids with Boehmite; American Chemical Society 4025-29 (1995)). Based upon this approach, the reaction of boehmite, [Al(O)(OH)]n, with carboxylic acids, has been developed (Landry et al., From Minerals to Materials: Synthesis of Alumoxanes from the Reaction of Boehmite with Carboxylic Acids; 5 Journal of Materials Chemistry 331 (1995) (hereinafter, xe2x80x9cLandry et al. (1995)xe2x80x9d) for Equation (1) below. 
Carboxylate-substituted alumoxanes have been well characterized (Landry et al. 1995 and (Callendar, R. et al.; Aqueous Synthesis of Water-Soluble Alumoxanes: Environmentally Benign Precursors to Alumina and Aluminum-Based Ceramics; American Chemical Society 2418-33 (1997) (hereinafter, xe2x80x9cCallendar et al. (1997)xe2x80x9d). Solution particle-size measurements show that carboxylate-alumoxanes are nano-particles, with sizes ordinarily ranging from 1-1000 nm. The carboxylate ligand is bound to the aluminum surface, and is only removed under extreme conditions. The carboxylate-alumoxane materials prepared from the reaction of boehmite with carboxylic acids are air and water stable materials that are easily processable. The physical properties of these alumoxanes are highly dependent on the identity of the alkyl substituents, R, and range from those associated with insoluble crystalline powders to powders that readily form solutions or gels in hydrocarbon solvents and/or water. These alumoxanes are indefinitely stable under ambient conditions, and are adaptable to a wide range of processing techniques. The alumoxanes can be easily converted to aluminum oxide upon mild thermolysis, while they also react with metal complexes to form doped or mixed aluminum oxides (Kareiva, A. et al.; Carboxylate-Substituted Alumoxanes as Processable Precursors to Transition Metal-Aluminum and Lanthanide-Aluminum Mixed-Metal Oxides: Atomic Scale Mixing via a New Transmetalation Reaction; American Chemical Society 2231-2340 (1996) (hereinafter, xe2x80x9cKareiva et al. (1996)xe2x80x9d).
Apblett et al. (1992) describes a method for the synthesis of carboxylate-substituted alumoxanes through the reaction of pseudoboehmite and carboxylic acids in refluxing xylenes. If the carboxylic acid is a heat stable liquid, then the carboxylic acid can be prepared by addition of the pseudoboehmite to the liquid acid with reflux heating for several days. Apblett et al. (1992) describes the synthesis of several alumoxanes using different carboxylic acids (HO2CR, Rxe2x95x90CH3, C5H11, CH2Cl, C7H15, CH2CH2OCH3, or CnH2n+1 where n=1-3). However Apblett et al. (1992) do not describe the preparation of carboxylate-alumoxanes containing functional groups that can be incorporated into polymer matrices through covalent bonding.
Landry et al. (1995) expand on the synthesis and characterization of carboxylate-alumoxanes including those listed above and additionally describe new carboxylate-alumoxanes prepared using valeric and lauric acids. However, Landry et al. (1995) do not describe the synthesis and use of alumoxanes containing functional groups that can be readily incorporated through chemical bonding to polymer matrices. Kareiva et al. (1996) describe the preparation of carboxylate-alumoxanes and a metal-exchange reaction using metal acetylacetonate compounds to prepare metal exchanged alumoxanes that can be decomposed in air at high temperatures to prepare mixed metal oxides. A detailed discussion of metal-exchanged alumoxanes can be found in U.S. application Ser. No. 09/058,587 filed Apr. 10, 1998 and titled Metal-Exchanged Carboxylato-Alumoxanes, now U.S. Pat. No. 6,207,130, incorporated herein by reference.
The synthesis of two alumoxanes not included by Apblett et al. (1992) and Landry et al. (1995) were included in Kareiva et al. (1996). These alumoxanes were prepared by the reaction of pseudoboehmite with methacrylic acid and (methoxyethoxy)ethoxy acetic acid respectively. Although methacrylato-alumoxanes contain a reactive group that could potentially be incorporated into polymers through free radical addition mechanisms, they were used as precursors to the formation of mixed metal oxides and Kareiva at al. (1996) did not discuss the use of these materials as potential candidates for incorporation into polymer matrices. Callender et al. (1997) describe the synthesis of carboxylate-alumoxanes by reactions of carboxylic acids and pseudo-boehmite using water as a solvent. Syntheses of carboxylate-alumoxane from boehmite and the carboxylic acids (methoxyethoxy)ethoxy acetic acid, (methoxyethoxy) acetic acid, methoxy acetic acid and acetic acid were described. However, these materials were prepared to be used as precursors in the production of ceramics and do not contain functional groups that can be used to covalent bond the alumoxanes to a polymer matrix. In summary, the syntheses and uses of carboxylate-alumoxanes cited in the above works were directed towards their use as precursors that are thermally decomposed to provide oxide ceramics and powders.
Hence, it is desirable to provide a technique for synthesizing alumoxanes, and in particular carboxylate-substituted alumoxanes, and incorporating them as fillers into various thermosetting and thermoplastic polymers.
The present invention provides methods and materials for covalently bonding nano-sized inorganic fillers comprising alumoxanes into a wide range of polymer matrices. More specifically, the present invention relates to the substitution, completely or partially, of one or more of the constituents of an organic or inorganic polymer system with a carboxylate-alumoxane that is chemically functionalized with appropriate substituents to allow for chemical bonding between the carboxylate-alumoxane and the polymer.
The invention describes, and provides methods for the preparation of, carboxylate-alumoxane/polymer composite and hybrid materials through the reactions of amine, hydroxyl, acrylic and vinyl substituted carboxylate-alumoxanes with low molecular weight polymer precursors containing the appropriate reactive functional groups, such as oxiranes, acid chlorides, anhydrides, carboxylic acids, quinones, and olefins. These reactions lead to incorporation of the carboxylate-alumoxanes into the polymer matrix through chemical bonding between the carboxylate-alumoxane and the polymer. In an alternative embodiment, more than one type of substituted carboxylate-alumoxane, with or without additional low molecular weight polymer precursors may be employed, such that the different reactive substituents on each of the substituted carboxylate-alumoxanes react with each other, thus forming chemical bonding between the different carboxylate-alumoxanes and any polymer present. The self-reaction of the reactive substituent on the carboxylate-alumoxane may also lead to an alumoxane-based polymer.
The inventive method is based on the use of chemically functionalized carboxylate-alumoxanes that can be described by the general formula:
[Al(O)x(OH)y(O2Cxe2x80x94Rxe2x80x94X)z]n
and/or
[Al(O)x(OH)y(O2Cxe2x80x94Rxe2x80x94X)z(O2CRxe2x80x2)zxe2x80x2]n, etc.
where Xxe2x80x94Rxe2x80x94CO2xe2x88x92 and Rxe2x80x2xe2x80x94CO2xe2x88x92are mono-carboxylates, R and Rxe2x80x2 are from the group of a hydrogen and/or an organic group, and X is a chemically reactive substituent. The organic group is preferably an alkyl, alkenyl, aromatic, haloalkyl, haloalkenyl, haloaromatic group or alkyl, alkenyl, aromatic ether group or an organic group containing a hetero-atom including, oxygen, nitrogen, sulfur, or phosphorous. The chemically reactive substituent is preferably a hydroxyl, amine, acrylate, vinyl, olefin, or similar chemical substituent. These components can be prepared by the methods described in Landry et al. (1995), Apblett et al. (1992), Kareiva et al. (1996), or the preferred method of Callender et al. (1997), all of which are incorporated herein by reference. The composition of the carboxylate-alumoxane varies depending on the starting materials employed and the details of the synthetic method employed, as set out in Callender et al. (1997).
The present disclosure describes the use of amine, hydroxyl and acrylic functionalized carboxylate-alumoxanes to prepare a wide range of alumoxane-polymer composites. The classes of alumoxane polymers described herein include but are not limited to: epoxies, phenol-formaldehyde resins, polyamides, polyesters, polyimides, polycarbonates polyurethanes and quinone-amine polymers. Thus, the present invention includes the syntheses of chemically functionalized carboxylate-alumoxanes containing aromatic and aliphatic amines and aromatic and aliphatic hydroxyls and their incorporation in a range of polymer matrices to produce alumoxane-polymer composites.